The invention is in the field of x-ray machines, such as those used for chest radiography and shadowgraphic radiography of other parts of a patient's body or of an inanimate object. Its main object is to improve image quality, for example by ensuring that just the right amount of radiation is used to accomplish a desired image characteristic, be it a desired image density or contrast or signal-to-noise (S/N) ratio or some other characteristic. For example, in imaging both lungs and mediastinum in a single picture, just the right amount of radiation can be delivered to achieve the required diagnostic content. The receptor can be conventional x-ray film or it can be some other receptor, such as a digital or a digitized receptor. The advantages of the invention include, in the case of film receptors, overcoming sensitometric limitations and, in the case of both film and digital receptors, control of S/N degradation from transmitted primary field variations, control of both the noise degradation and the nonlinear effects of scattered radiation, regaining low spatial frequency information lost initially by equalization, and reduction of exposure period and scanning artifacts.
Shadowgraphic radiography has been widely used for many decades, and has long-recognized inherent limitations. For example, chest radiography is probably the most frequently performed x-ray examination in a typical radiology department, and tens of millions of chest x-rays are taken annually in this country alone. However, in spite of its clinical importance it is far from being a technically consistent procedure and is subject to large variations in image quality, sometimes with imperfections in clinical results. One reason is that the posterior-anterior and lateral projections of the chest pose significant challenges. The presence of scattered radiation reduces film contrast, even when anti-scatter grids are used or an effort is made to reduce the scatter component by the use of air gap techniques or sophisticated anti-scatter grid designs. Scanning slit devices have also been used, bringing about significant contrast improvement in imaging the head, abdomen, chest and breast. Another inherent limitation in conventional chest radiography arises from the wide variation in patient x-ray thickness (meaning attenuation along a given raypath) between the lung field and the relatively thick mediastinal, retrocardiac and diaphragmatic portions, which produces a large variation in receptor exposure. One aspect of this is sensitometric, in that it may not be possible to achieve proper density or contrast at the portions of the image which may be of interest. Another deals with S/N ratios, in that the radiation reaching some portions of the receptor may be too little, in which case the S/N ratio can be too low, and that reaching other portions may be more than enough, in which case the patient would be exposed to more radiation than needed. Efforts to improve the exposure range of radiographic film with wide latitude films offer a wider exposure range, however at the expense of contrast in the lung fields, and with reduced signal-to-noise ratio over the thicker, underexposed portions. One approach to rectify these exposure problems is through the use of portal x-ray compensation filters shaped to match the contour of the lung fields and preferentially attenuate the pre-patient x-ray beam over the lungs, resulting in a more uniform film exposure. The obvious limitation is the difficulty of designing a filter to match the large variations in lung contour and patient thickness expected in a typical patient population. The so-called unsharp mask technique addresses this limitation by using a tailored optical filter for each patient, to be used in the film cassette during exposure. While these techniques can produce images with improved contrast uniformity, they can be time consuming and prone to misregistration artifacts in their clinial application. Digital radiography using arrays of detectors having wide dynamic range represents another imaging technique which offers good scatter rejection, image contrast control, and a potential for image data manipulation using temporal subtraction and multiple imaging techniques. However, in the systems known to the inventor herein these improvements are gained at the expense of spatial resolution, x-ray tube heat loading increase and an increased system complexity and cost.
Significant improvements have been made through the use of scanning equalization radiography using both prepatient and post-patient collimation to reduce scatter and a feedback technique to modulate a scanned x-ray beam. See, e.g.: Plewes, D. B., Computer-Assisted Exposure In Scanned Film Radiography, Proceedings International Workshop On Physics And Engineering In Medical Imaging, March 1982, pp. 79-85; Wandtke, J. C. and Plewes, D. B., Improved Chest Radiography With Equalization, RadioGraphics, Vol. 3, No. 1, March 1983, pp. 141-154; Plewes, D. B., A Scanning System For Chest Radiography With Regional Exposure Control: Theoretical Considerations, Med. Phys. 10(5), September/October 1983, pp. 646-654; Plewes, D. B. and Vogelstein, E., A Scanning System For Chest Radiography With Regional Exposure Control: Practical Implementation, Med. Phys. 10(5), September/October 1983, pp. 655-663; Plewes, D. B. and Vogelstein, E., Exposure Artifacts In Raster Scanned Equalization Radiography, Med. Phys. 11(2), March/April 1984, pp. 158-165. The contents of said publications are hereby incorporated by reference in the specification as though fully set forth herein.
Despite the significant progress made in the past in improving image quality, it is believed that room remains for improvement. In an effort to meet at least some aspects of this need, one of the features of this invention is to reduce signal-to-noise ratio variations in the x-ray image. One way of doing this in accordance with the invention is by measuring both post-patient scatter and post-patient primary radiation and using the results in a feedback loop controlling the pre-patient x-ray beam. One benefit is that the patient tends to be exposed only to the amount of radiation needed to produce an image of a given quality. Another is an overall improvement in image quality.
It has been proposed in the past, in the context of CT scanners, to ensure adequate signal-to-noise ratio by concurrently monitoring the integrated radiation from all of the detectors for a given x-ray pulse and ending the pulse only when all detectors have received at least a threshold quantity of radiation believed sufficient for an adequate signal-to-noise ratio. See U.S. Pat. No. 4,260,894. One of the many differences between this prior art proposal and this aspect of the invention disclosed and claimed here is that in the invention here each spot of the x-ray receptor should receive only the radiation sufficient for a selected signal-to-noise ratio, while in the prior art proposal it is only ensured that each CT detector would receive no less than the amount of radiation needed for a satisfactory signal-to-noise ratio (but in fact many detectors are likely to receive more, and thus expose the patient to more radiation than needed). Other proposals for modulating the pulse width of the pre-patient x-ray beam to achieve uniform film darkening are discussed in the publications authored or co-authored by Dr. Plewes, the inventor herein, cited above.
Another feature of the invention disclosed and claimed here is improving image quality by dynamically modulating each of the pre-patient intensity and pre-patient hardness of the x-ray beam on the basis of intermittent post-patient beam measurements made at selected positions of the beam relative to the patient and at a selected low beam intensity. It is generally desirable to use high KV (harder) radiation to reduce the dynamic range requirements on the receptor but to use low KV (softer) radiation in certain parts of the body (e.g., the lungs) to increase contrast. It is also generally desirable to use high intensity radiation through highly attenuating parts of the body (e.g., bone), so as to get sufficient radiation to the receptor, but low intensity through low attenuation parts of the body (e.g., lungs and soft tissue) to increase contrast. In this aspect of the invention disclosed and claimed herein, a very short pulse of low energy radiation and a very short pulse of high energy radiation are used at each selected beam position while scanning the patient. The relative amounts of bone and soft tissue along each beam are determined from those short pulses, and the best combination of intensity and hardness for that beam position is found and the x-ray tube is energized accordingly.
It has been proposed in the prior art (see U.S. Pat. No. 4,032,784) to control both the x-ray tube current and voltage in a raster scan x-ray machine so as to improve the picture and reduce radiation exposure. The patent proposes dynamically varying beam intensity to have high intensity through highly attenuating parts of the body and low intensity otherwise, and concurrently dynamically varying KV to have harder (shorter wavelength) radiation through high attenuation parts of the body and softer x-rays otherwise. However, the prior art patent proposes deriving the control signal from the output of the detector in the normal scanning operation rather than from short preliminary bursts of radiation. Another, similar prior art proposal is U.S. Pat. No. 2,962,594.
Another nonlimiting aspect of the invention relates to ensuring constant line velocity when raster scanning a patient through the use of a special, curved slit, rotating pre-patient collimator. While it is possible to use linearly moving collimator apertures (or an x-ray tube) to ensure constant line velocity (and thus facilitate modulation techniques) it is mechanically more efficient to use a rotating collimating aperture. This, however, introduces variation in the beam velocity within a raster scan line and complicates beam modulation.
A single x-ray beam can be raster scanned in overlapping scan lines to produce an x-ray image. This, however, must take several seconds, which can lead to motion artifacts. In order to reduce scanning time, and hence motion artifacts, another feature of this invention is to use a segmented fan beam scanned across the patient in a direction transverse to the plane of the fan. In accordance with a nonlimiting aspect of the invention, the fan segments are individually dynamically modulated to improve the image and reduce patient dosage. While there have been prior art proposals for using fans segmented by pre-patient or post-patient collimators, or both, as for example in the so-called localizer mode of CT scanners, and shaped collimators can be used to vary the pre-patient attenuation as between fan segments, the invention disclosed and claimed herein adds the benefit of dynamically and individually modulating the segments through a feedback loop.
There are at least two general application of equalization at this time, namely to x-ray film (nonlinear systems) and to wider dynamic range receptors, such as digital or digitized receptors. Especially for wider dynamic range receptors, the invention offers benefits such as control of signal/noise degradations from transmitted primary field variations, control of both the noise degradation and the nonlinear effects of scattered radiation, regaining low spatial frequency information lost by equalization and reduction of exposure period and of scanning artifacts.
With respect to control of signal/noise degradations from transmitted primary field variations, the objective is to maintain an approximately constant noise structure throughout the image, which implies approximately constant receptor exposure. Both primary and scattered radiation are important, and thus in accordance with the invention control can be maintained of both the noise degradation and the nonlinear effects of scattered radiation. While it is possible to use very narrow beam widths (e.g., 1 mm or less) to reduce scatter contamination, this may not be clinically practical because it places a severe load on the X-ray tube, as most of the radiation is blocked by the collimator. A more practical approach is to use a scanning beam which is a few cm wide, which cuts the X-ray tube load requirements but also increases scatter. Beam equalization in accordance with the invention offers two benefits in this respect. First, since the primary radiation levels at the receptor can be maintained approximately constant, the scatter/primary ratios will tend to be nearly constant. This can allow a measurement of scatter by looking beside the primary beam to be a good approximation of the full scatter profile. Thus, in accordance with the invention the scatter field can be measured while scanning to generate an approximate scatter field map which later can be subtracted from the initial image. This can make the scatter data more suitable for dual energy imaging and image processing techniques, which tend to be particularly sensitive to scatter contamination. This approach can allow a good approximation of the correction needed to account for the nonlinear effects of scatter, although the noise due to scatter would still be present. A second aspect of this approach is to use the scatter measurements made during the scan to adjust the x-ray tube output to compensate for the noise degradation from scatter.
With respect to regaining low spatial frequency information lost by equalization, it should be clear that one of the reasons equalization achieves improved images is that it rejects low frequency subject contrast. For example, the contrast between the mediastinum and the lung field is nearly eliminated by equalization. While this is useful in many, if not most, clinical situations, it can be troublesome in those where disease is manifest by low contrast variations that are diffuse and without sharp edges. Pneumothorax is a case in point. In this regard, the digital application of equalization can be particularly useful, because a record can be made during the scan of the spatial distribution of patient exposure. This information can then be used to correct the recorded data set to regain the lost low frequency information that would have been present in the uncorrected, equalized image. A simpler but less accurate way to do this is to normalize the measurements made for a one dimensional scan of the x-ray beam across the patient. If a fan beam is used, it is preferable to orient it vertically, i.e., to have its plane parallel to the mediastinum. Another way is to normalize the measurements for a two-dimensional scan. Here the two-dimensional distribution can maintain more accurately the noise uniformity over the image.
With respect to reduction of exposure period and scanning artifacts, a digital receptor can reduce the need for overlapping the scanning beams and thereby significantly reduce the exposure period. This can be done without very precise mechanical scan line registration, which is essential with film in order to prevent scan line artifacts, because with a digital receptor the line spacing can be made periodic and the scan line artifacts can be numerically filtered out.
Thus, in the case of digital receptors the features of the invention discussed above can lead to equalized images which are substantially free from scanning artifacts, can be produced in short exposure periods (e.g., of the order of 35-50 mS) to reduce motion blurring, can exhibit all desired low frequency structures and this can be applied to both one-dimensional and two-dimensional equalization systems, can have an approximately constant SN ratio, and can be free of significant scatter contamination.
These and other aspects of the invention are explained in greater detail in connection with the figures described below.